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PhysicsMadrid 2026Question 4.AFree lesson

The photoelectric effect on a silver plate

Two lights and two stopping potentials: why they give us the Planck constant and the work function

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Problem statement

Question 4.A.- When a silver plate is illuminated with light of 150 nm, a potential of 3.55 V is needed to stop the emitted electrons. However, if light of 200 nm is used, the potential needed to stop the electrons is 1.48 V. From these data, determine:

a) (1.5 marks) The value of the Planck constant.

b) (1 mark) The work function of the silver plate, in eV.

Data: Speed of light in a vacuum, $c = 3\cdot10^{8}\ \text{m s}^{-1}$; Absolute value of the electron charge, $e = 1.6\cdot10^{-19}\ \text{C}$.

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · Physics · Question 4.A. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.

What this lesson answers

  1. What does the question tell us before we calculate anything?
  2. Where does Einstein's equation come from?
  3. Why does the stopping potential measure the maximum kinetic energy?
  4. Why do we need two lights?
  5. a) What is the value of the Planck constant?
  6. a) Another route: the line of potential against frequency
  7. b) How much does it cost to pull an electron out of silver?
  8. The two answers and how to check them
  9. Common mistakes to avoid
  10. Exam tips
  11. For top marks

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